This invention relates to a feedback control method of controlling the air-fuel ratio of a mixture being supplied to an internal combustion engine, and more particularly to a method of this kind, which is applied when the engine is operating in a predetermined accelerating condition in the course of transition into a feedback control effecting region.
A fuel supply control method for an internal combustion engine, particularly a gasoline engine, has been proposed, e.g. by U.S. Pat. No. 4,445,482 issued May 1, 1984, which determines the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air-fuel ratio of an air-fuel mixture being supplied to the engine, by first determining a basic value of the valve opening period as a function of engine rotational speed and intake pipe absolute pressure and then correcting the basic value through addition of constants and/or multiplication by coefficients, which are functions of engine operating parameters such as engine rotational speed, intake pipe absolute pressure, engine coolant temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed method, while the engine is operating in a normal operating condition, the air-fuel ratio is controlled in closed loop or feedback mode such that the valve opening period of the fuel injection device is controlled by varying the value of a coefficient in response to the output of an exhaust gas ingredient concentration sensor (O.sub.2 sensor) which is arranged in the exhaust system of the engine, so as to attain a stoichiometric mixture ratio or a value close thereto, whereas while the engine is operating in one of particular operating conditions (e.g. a mixture-leaning region, a wide-open-throttle region, and a fuel-cut effecting region), the air-fuel ratio is controlled in open loop mode by the use of a mean value of values of the above coefficient applied during the preceding feedback control, together with an exclusive coefficient corresponding to the kind of the particular operating region in which the engine is then operating, thereby preventing deviation of the air-fuel ratio from a desired air-fuel ratio suitable to the particular operating region due to variations in the performance of various engine operating condition sensors and a system for controlling or driving the fuel injection device, etc., which are caused by machining tolerances or the like and/or due to aging changes in the performance of the sensors and the system, and also achieving required air-fuel ratios best suited for the respective particular operating conditions, to thus reduce the fuel consumption as well as improve the driveability of the engine.
However, according to the proposed method, when the engine is accelerated from an idling condition while it is operating in the air-fuel ratio feedback control effecting region, the feedback control based upon the coefficient dependent upon the O.sub.2 sensor output has a response lag, that is, there is a delay in the increase of the fuel supply quantity that should take place with increase of the intake air quantity caused by opening of the throttle valve so that the air-fuel ratio of the mixture supplied to the engine becomes leaner than the desired value or stoichiometric mixture ratio. More specifically, the mixture instantly becomes leaner immediately after the intake air quantity is increased due to opening of the throttle valve for acceleration of the engine. However, the feedback control is such that so long as the output of the O.sub.2 sensor remains either higher than a reference value corresponding to the desired air-fuel ratio, or lower than same, the fuel supply quantity is controlled by integral control, to vary at a small rate. As a result, the air-fuel ratio remains leaner for a considerable time period after engine is accelerated, whereby a considerable amount of NOx is emitted.